Basically it is as it sounds, cleansing twice. This is a great way of getting you face really clean.
How many are using, one cleanser, and then a toner on a cotton pad. Then you look at the cotton pad, is it clean? No, most often it is not. What people, including me (before Korean skincare was all on my brain) did, was using toner as your last step. Korean toners are so much more than that and should not be used as a last cleansing of the residue step. You can read about Korean toners here